In switched connection communications networks, an end-to-end connection or call is established and maintained by one or more on-demand bearer channels which together define a bearer channel path across the network being traversed. Examples of switched connection communications networks are the typical public switched telephone networks and virtual connection oriented digital networks in the form of Asynchronous Transfer Mode (ATM) or Frame Relay (FR) networks. In the ATM networking environment, an end-to-end call may take several forms. For instance, such calls may be provisioned by the use of a switched virtual connection (SVC) or a soft permanent virtual connection (S-PVC), described more fully below. Generally, these types of connections may be dynamically established and released substantially in real time by network elements such as data transmission switches, all in accordance with standard signalling protocols well known to those skilled in this art. An example of one such network element which allows for the dynamic initiation and release of calls is the model 36170 MainStreetXpress™ ATM multiservices network switch, which is commercially available from Newbridge Networks Corporation of Kanata, Ontario, Canada.
Network connections in the form of the previously mentioned SVC or S-PVC varieties are set up on demand, unlike a permanent virtual connection (PVC). The PVC is an end-to-end network connection that is defined at subscription time and is normally provisioned by a network management entity which typically operates over a management interface into the switches of the end-to-end path. Thus, PVC type connections are usually configured as relatively long hold time connections. With SVC type connections, on the other hand, these are set up on demand via a signalling protocol and tend to be associated with relatively short hold times compared to PVC type connections. An S-PVC is a signalled PVC which is set up on demand via a signalling protocol. Thus, an S-PVC is a kind of hybrid connection sharing some of the characteristics of each of a PVC and an SVC.
Ordinarily, signalling between network elements is carried over a signalling network comprising a call control and processing infrastructure which is associated with each network element, and further comprising an interface for communicating between counterpart network elements. The interface can consist of a separate overlay network, such as leased lines, as may be found in a Frame Relay SVC service. More typically, however, the interface consists of a PVC which has been dedicated for the transfer of signalling information or call control data between interconnected network elements. Such signalling virtual circuits can be carried over the same internode link facility as the bearer channels, or on separate dedicated links.
In order to initiate an end-to-end call, a calling device will typically transmit a call establishment request message to the network indicating a destination address for the call and the desired connection and quality of service parameters. For SVC type connections, the calling and called devices are typically customer premise equipment (CPE). For S-PVC type connections, the calling and called devices are typically ingress and egress network elements, as described in greater detail below. The call establishment request message is propagated through the network to a called device or destination address using one of two conventional routing techniques, namely hop-by-hop routing or source routing. In hop-by-hop routing, each network element which receives the call setup request message typically consults a routing table in order to determine the next hop or output bearer channel towards the intended destination. In source routing, on the other hand, the source or ingress network element maintains a database of the topology of the entire network and specifies the output ports that each network element should use to route the call.
Each network element which receives a call establishment request message generates a bearer channel cross-connect which links an input bearer channel to an output bearer channel. Ultimately, the call set up request message is relayed by the signalling network to the called device, and the called device is thus informed as to the identity of the bearer channel it should use for transmitting information in respect of the call. Once the call establishment request message is received at the destination device or destination address, a connection establishment message is sent back over the signalling network from the called device to the calling device in order to indicate that a connection has been established. At this point, the call is confirmed to have been established. The calling device, and the called device in the case of a bi-directional connection, may transmit user data through the network over the established bearer channel path. The signalling network may also be used to dynamically release a call and its associated bearer channel path in a manner similar to that used to establish the call, which was previously explained. When a call is to be released, the network entity initiating the release will typically transmit a call release message to the other device which is involved in the call. Call release may occur either by reason of a caller initiated termination or by reason of a network outage such as a signalling link failure or a port failure.
As well as being used for dynamically establishing and releasing a call, the signalling network is also used to transmit various types of status messages or link state messages relating to the operation of the call and its bearer channel path. These status or link state messages are associated with various sensing mechanisms employed by signalling standards for determining whether a given network entity or a link therefrom is alive and properly functioning. Such mechanisms typically include heartbeat processes relating to various layers of signalling protocol, as is described in greater detail below. A signalling link or port may fail for various reasons, including a software defect, an equipment failure in the call control infrastructure or a failure in the transmission link. When a failure is sensed as described above by other network elements adjacent to the failed portion of the signalling network, signalling standards will typically specify that all calls affected by the failure should be released.
When a signalling link or port fails, a signalling link layer which provides the functionality of order and flow control at each end of the failed link will detect that the link is no longer functioning. Pursuant to typical signalling protocols, this causes the signalling entities adjacent to the link failure to tear down or release all switched connections which utilize the failed signalling link or port. In both the upstream and downstream directions of a signalling link failure, all virtual connections which utilize the signalling link are typically released along the end-to-end path towards the end users. In the case of SVC type connections, these connections are cleared back to the source and destination of the calls, and end users must themselves proceed to re-establish the connections. In the case of typical S-PVC type connections, the signalled PVCs which are affected by the signalling link failure are automatically re-established after they are released in the manner described above to the calling party. Apart from signalling link and port failures, virtual connections may be released in a switched connection oriented network when a call is intentionally terminated by either the calling party or the called party, or due to an administrative action from a call processor or other management entity for the network environment.
Various differentiated levels of call service may be associated with connections over a switched network. In the prior art, the release of connections following a signalling link failure is typically carried out according to an arbitrary sequence. Thus, where connections with such differentiated levels of service share a given signalling link or port, it is possible for a connection that is associated with a superior level of service to be released later in sequence than a connection that is associated with an inferior level of service. It is also possible that the release of the connection having an inferior level of service will propagate to its source earlier when compared to the later released connection having the superior level of service. In either case, then, there is a likelihood that the connection with the lower level of service will be re-established over an alternate connection path prior to the connection with the higher level of service. If network resources are scarce or constrained, such a result could be unfavourable or unfair to the user of the connection with the higher level of service, especially if such user were charged a premium for call service that is expected by the user to deliver better performance.
The propagation delay associated with the transmission of a connection release message from a network node immediately adjacent to a network outage to a network node constituting a source or a destination for the connection will vary based upon the number of network nodes across which the connection must be released and the speed of the links connecting those nodes. All things being equal, it can typically be expected that multiple connections which are individually released from the same network node in a given sequence will have a general tendency to complete propagation to their respective source or destination nodes in more or less the same sequence. However, this will not always hold true for all network topologies.
Existing ITU-T specifications for communications networks describe the assignment of call priority to various connections in a network so as to allow for the preferential treatment of higher priority calls during call establishment. This is based on a priority level which has been preassigned to the call. These specifications detail the manner of handling a call in priority sequence during the call establishment phase of signalling, as outlined in the following publications: “Digital subscriber signalling system No. 2—Call priority”, Series Q: Switching and Signalling, ITU-T Q.2959 dated July 1996 and “Broadband ISDN—B-ISDN application protocols for the network signalling. B-ISDN User Part—Call Priority”, Series Q: Switching and Signalling, ITU-T Q.2726.2 dated July 1996.
It is a known technique in connection oriented networks to utilize inverse multiplexing in order to aggregate the capacity of a number of given physical links or trunks. For instance, in ATM networking, network trunks which are known to those in this art as IMA trunks are employed for this purpose. An IMA trunk is a logical trunk that is comprised of a plurality of smaller physical trunks. For instance, a number of T1 or E1 trunks may be multiplexed together to form a single logical cell stream of higher capacity, thereby allowing for data flows which may exceed the capacity of a given physical trunk. The use of multiple trunks also provides for redundancy of physical links such that if one trunk fails, connections may still be supported on the remaining trunks. The acronym “IMA” is used to describe inverse multiplexing over ATM networking environments.
With IMA logical trunks, a subset of the underlying physical trunks may fail, leaving the remaining underlying physical trunks of the logical trunk intact. This results in a reduced aggregate bandwidth for the overall logical trunk. Another situation may arise in which administrative actions, such as the removal of a physical link from a logical trunk, may reduce the aggregate available bandwidth of calls which are established over the logical trunk. When a reduction in available bandwidth occurs by reason of either of these causes or for other reasons, the calls utilizing the trunk may suffer a degradation in the quality of service unless the number of calls utilizing the logical trunk is reduced. This type of problem may occur in any communications technology where the physical layer or effective bandwidth can change over time or be reduced due to failures or physical impairments, for instance wireless or xDSL (Digital Subscriber Loop) technologies. In such situations in the prior art, calls which utilize the partially failed trunk are typically released in an arbitrary sequence. If a prioritized scheme for call release were to be implemented in the context of inverse multiplexing, this would permit the highest priority connections for a partially failed aggregated logical trunk to be retained for the remaining available bandwidth while releasing all other connections over the failed trunk in order of their priority sequence.